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The 2011 Nissan Leaf, which was delivered to its first owners this month, runs entirely on battery. But the Environmental Protection Agency says it can travel 99 miles on the equivalent of a single gallon of fuel.

Confused? You're not alone. The mileage-equivalent ratings, meant to help potential buyers compare electric cars with others in their class, are befuddling some consumers who see them as an automotive example of comparing apples and oranges.

And that's just one of the conundrums shoppers will face as more electric vehicles arrive in showrooms.

The EPA isn't the only entity comparing the clean-fuel cars with autos that have traditional internal combustion engines. Their ratings, which will be posted on car windows, may end up sharing space with stickers from the Federal Trade Commission and the automakers.

That's some crowded glass.

"It's a whole new world that needs to be rated," Nissan spokeswoman Jeannine Ginivan said. "For now, the consumer is going to have to decipher everything and see how to make it work for them."

Traditionally, the fuel economy rating for vehicles with internal combustion engines is calculated from emissions generated during a series of tests. Things got hairy with the Leaf. The EPA worked out a formula in which an electric car using 33.7 kilowatt-hours of electricity was considered equivalent to a standard vehicle using a gallon of gasoline. Because there wasn't any physical fuel, the rating was renamed miles per gallon equivalent, or mpge.

The electric vehicle will be sold with a sticker saying that it can reach up to 106 mpge in the city. The previous record-holder, the Toyota Prius hybrid, was rated at 51 mpg in urban settings.

The Leaf score is straightforward compared with the one given to the 2011 Chevrolet Volt, a hybrid plug-in that switches to gasoline when its electricity supply runs out. The Volt doesn't just have an overall rating. In addition to an umbrella figure of 60 mpge, the government gave it a 93-mpge rating for when the Volt is driven only in its electric mode and a 37-mpg figure for when it uses gasoline only. But few, if any, Volt drivers will stick to just one mode.

The EPA figures also include expected tailpipe emissions. Since it doesn't need a tailpipe, the Leaf will release no carbon dioxide, according to the agency.

Some environmentalists aren't pleased with the prognosis and suggest using what's known as a well-to-wheel analysis instead. The process would consider all the greenhouse gases released from the time the electricity is first generated until it is sent through transmission lines to charging units.

Based on such measurements, the Leaf would rack up more than 250 grams of carbon dioxide and other emissions every mile, according to data from the Energy Department's Argonne National Lab. Gasoline-fueled cars on average release 450 grams a mile.

"It doesn't matter where greenhouse gas emissions are emitted, because they'll still affect climate change around the world," clean-energy consultant Sandy Thomas said. "The fact that the emissions came from a coal plant producing electricity in Utah is just as bad as if they came out of the tailpipe."